1. Field of the Invention
The present invention relates to a fuel injection pump. For instance, the present invention can be suitably applied to a fuel injection pump used in an accumulation type fuel injection system of a diesel engine.
2. Description of Related Art
There is a fuel injection pump having a camshaft, a cam ring and at least one plunger, for instance, as disclosed in Unexamined Japanese Patent Application Publication No. 2002-364480 (Patent Document 1, hereafter) or No. 2002-250459 (Patent Document 2, hereafter). The camshaft has a cam, which has a circular section, thereon. The cam ring is rotatably fitted to an outer periphery of the cam through a bush. The plunger is held inside a cylinder so that the plunger can reciprocate in the cylinder. If an engine drives the camshaft to rotate, the rotational movement of the cam is transmitted to the plunger through the cam ring. Thus, the plunger reciprocates inside the cylinder and pressure-feeds the fuel. The fuel injection pump has two fuel pressurizing chambers, which are alternately pressurized by the two reciprocating plungers. The fuel injection pump has discharge valves for alternately discharging the fuel pressurized in the fuel pressurizing chambers.
There is a possibility that extraneous matters are mixed into the fuel and get stuck between operating members, which perform rotational movement, reciprocating movement, and the like.
The fuel injection pump disclosed in Patent Document 1 includes a rotary pump for supplying low-pressure fuel into the fuel pressurizing chamber. An inner rotor of the rotary pump is screwed to the camshaft at a predetermined torque through a bolt having a lead directed in the same direction as the rotation direction of the camshaft. If the extraneous matters in the fuel get stuck between gears of the inner rotor and an outer rotor, an abnormal turning force will be generated in the camshaft. In this case, the abnormal turning force will overmatch a force fastening the bolt, and the bolt will be loosened. As a result, the camshaft and the inner rotor are uncoupled.
The fuel injection pump disclosed in Patent Document 2 includes a suction quantity control electromagnetic valve for supplying the fuel into the fuel pressurizing chamber and for controlling the quantity of the fuel pressurized and pressure-fed by the plunger. A valve member and an armature of the suction quantity control electromagnetic valve are formed with penetration passages axially penetrating the valve member and the armature. The suction quantity control electromagnetic valve is formed with a communication passage for connecting an upstream passage of control fuel with an armature chamber. Since a flow of the fuel is generated in the armature chamber, the fuel will not stay around the armature. Therefore, even if the extraneous matters included in the fuel exist in the armature chamber, the extraneous matters will be discharged outward along the flow of the fuel.
Usually, a filter is attached to a fuel inlet portion of the fuel injection pump in order to prevent the entry of the extraneous matters in the fuel from the outside.
The conventional technology can prevent defective operations or damages caused by the extraneous matters included in the fuel but cannot eliminate the extraneous matters sufficiently. The filter disposed in the fuel inlet portion of the fuel injection pump alone cannot sufficiently eliminate the extraneous matters, which can cause the defective operations or the damages.
There is a possibility that the extraneous matters such as burrs or chips generated during the manufacturing of components of the fuel injection pump remain inside. Therefore, the remaining extraneous matters are eliminated through cleaning and the like after the manufacturing. However, a housing has relatively complicated fuel passages among the components. Therefore, actually, there is a possibility that the extraneous matters remain in the fuel passages of the housing because of insufficient cleaning in high-pressure cleaning and the like performed after the manufacturing.
If the extraneous matters remaining because of the insufficient cleaning get stuck in a seat portion of a suction valve or a discharge valve as an operating member, fluid-tightness of the seat portion cannot be maintained and an appropriate fuel pressure-feeding quantity (a discharging quantity) cannot be obtained. If the extraneous matters get stuck in the seat portion of one of the discharge valves, which alternately discharge the fuel pressurized in the two fuel pressurizing chambers, and if the discharge valve is brought to a continuously opened state, the high pressure of the pressurized fuel is continuously applied to the plunger. As a result, poor lubrication will be caused between the plunger and a plunger sliding hole and seizing in the plunger will be caused. If the high pressure is continuously applied to the plunger, an excessive thrust force is applied to the cam ring. In this case, there is a possibility that the plunger breaks.
Moreover, in the case where the fraction produced when the plunger breaks moves inside a cam chamber and gets stuck between the housing and the cam ring, the housing will be damaged if the housing is made of aluminum.